In an induction motor arranged to rotate at a speed lower than a rotating magnetic field or rotate with a slip, it is known to control the slip frequency, so as to variably control the rotation speed of the motor. Also known is to perform vector control for adjusting an excitation current supplied to the stator and the magnitude and direction of a secondary current supplied to the rotor, to thereby improve dynamic operation and control characteristics of the motor.
FIG. 4 shows, by way of example, a conventional vector control system for a 3-phase induction motor, wherein a 3-phase induction motor 6 is provided with a speed detector 7 for detecting an actual rotation speed .omega.r of the motor, and current detectors CTU, CTV and CTW for detecting actual currents of respective phases of the motor.
In the control system of FIG. 4, the difference between a speed command Vc read out from a program (not shown) by a vector control processor (not shown) and an actual speed .omega.r detected by the speed detector 7 is amplified by an amplifier 1, to thereby derive a torque command T. This command T is divided in an element 2 by an excitation magnetic flux command .PHI. supplied from an element 8 to derive a secondary current command I2. Then, the product of a proportional constant K2 and the secondary current command I2 is divided in an element 10 by the excitation magnetic flux command .PHI. to derive the slip frequency .omega.s. More specifically, the element 10 is comprised of hardware. For example, the element 10 consists of two frequency dividers whose frequency dividing ratios are respectively set to values K212 and 1/.PHI. in a software fashion, and in this case, the frequency of a reference clock signal for the vector control is divided in these frequency dividers, to thereby derive a slip frequency .omega.s (=K212/.PHI.).
Further, in the control system of FIG. 4, the slip frequency .omega.s and the actual speed .omega.r are added together by an adder 11 to derive an excitation magnetic flux frequency .omega.0, and the excitation magnetic flux command .PHI. is divided in an element 9 by a proportional constant K1 to derive an excitation current component I0. A current calculation circuit 3 determines a primary current I1 on the basis of the excitation current component I0 and the secondary current command I2, and a 3-phase converter 4 determines current commands for the respective phases IU (=Il.times.sin.omega.0t), IV (=I1.times.sin(.omega.0t-2.pi./3)) and IW (=I1.times.sin(.omega.0t-4.pi./3)) on the basis of the primary current command I1 and excitation magnetic flux frequency .omega.0. Furthermore, a current controller 5 effects current control so that the differences between actual currents detected by the current detectors CTU, CTV and CTW and the current commands IU, IV and IW are reduced to zero.
However, according to the aforesaid conventional vector control system whose slip frequency determining element 10 is comprised of hardware, there occurs a slight slip frequency .omega.s attributable to characteristics of the hardware even when the secondary current command I2 is zero. Therefore, an error occurs in the slip frequency control. Further, since the reference clock signal is divided by the two frequency dividers after the frequency dividing ratios of the frequency dividers are set to K212 and 1/.PHI. in accordance with the secondary current command I2 and excitation magnetic flux command .PHI., a certain time period is required from an instant at which the commands I2 and .PHI. are generated to an instant at which a slip actually occurs. Therefore, the response in the slip control is lowered.